Combustion of coal can cause considerable air pollution from sulfur oxides and particulates of carbon and uncombusted, condensable hydrocarbons commonly referred to as "smoke." There are methods of suppressing the pollutants, but those techniques are primarily directed to methods of capturing the sulfur chemically or mechanically after high-sulfur coal is burned and before it leaves the stack. Another technique may be found in U.S. Pat. Nos. 2,824,047; 2,927,063; and 3,117,918, where the patentees mix carbonaceous solid fuels containing sulfur with a solid material capable of absorbing H.sub.2 S. The mixture is treated with hydrogen gas at a temperature of above 1100.degree. F. whereby the hydrogen gas combines with the sulfur to form H.sub.2 S. The H.sub.2 S is absorbed in situ by the H.sub.2 S acceptor. Such methods are costly and even though the United States has abundant deposits of high-sulfur coal, environmental regulations prohibit the burning of high-sulfur coal without adequate treatment of the gases.
Coal is very complex carbonaceous fuel containing various percentages of carbon, hydrogen, and sulfur fuel constituents, along with minor parts of oxygen, nitrogen, and ash as nonfuel mineral matter. Under conditions of high temperature pyrolysis, the coal is cracked or decomposed into solid, liquid, and gaseous constituents as coked residues, coal oils, and coal gases. The coal oils and coal gases arise from the volatile matter of coal, and under conditions of combustion the volatile matter can cause condensable smoke as an air pollutant. Sulfur in coal largely originates from (1) pyrite, FeS.sub.2, an inorganic mineral of coal ash and (2) organic sulfur compounds such as mercaptans and thiophenes. Under conditions of pyrolysis, the organic sulfur compounds decompose to low density, gaseous sulfides and high-temperature conversion of pyrite, FeS.sub.2, produces elemental sulfur and FeS, the more stable form of pyrite. These subsequently react with hydrogen and CO of pyrolysis to form H.sub.2 S, COS, and some heavier organo-sulfur gases.